Organic solvents have been used in large amounts in the fields of paints, inks, coating agents, adhesives, and various treating agents for textiles and paper. However, the use of organic solvents has become increasingly problematic over the years due to growing concern over conservation of petroleum resources, prevention of environmental pollution, enforcement of regulations of organic solvent effluents, such as RACT, restrictions on organic solvents brought about by revision of fire laws, and intensifying regulations on solvent concentration in the work environment.
In view of the above, various polyester compositions, such as a high solid polyester composition, a nonaqueous polyester dispersion, an aqueous polyester dispersion, a polyester emulsion, and a solventless polyester composition, have been proposed, and many of these already have been put to use. The aqueous polyester dispersion is the most promising for general use because of its ease of handling.
While most of the resins currently in use are hydrophobic, the resins for use as primers must have even higher hydrophobicity, along with a high corrosion resistance, making dispersal or dissolution of the hydrophobic resins in water difficult. Further, once the hydrophobic resin is dispersed or dissolved in water, it is difficult to render a resulting film resistant to water, corrosion and weather. These technical problems are commonly seen in any resin, and polyester is no exception.
It is known that polyester is dispersed or dissolved in water by copolymerization with a hydrophilic raw material, such as a compound containing a metal base of sulfonic acid, a polyalkylene glycol or an aliphatic dicarboxylic acid, which may be copolymerized singly or in combination. Regardless of which method is used, a large amount of a hydrophilic raw material must be used to achieve excellent water-solubility or water-dispersibility, and such use results in a film with very poor water and corrosion resistance.
For example, Japanese Patent Examined Publication No. 40878/1972 proposes the use of a compound containing a metal base of sulfonic acid in a proportion of not less than 8% by mole of the entire acid component of a polyester and a polyethylene glycol in a proportion of not less than 20% by mole of the entire glycol component of the polyester. Japanese Patent Unexamined Publication No. 121336/1975 also proposes the use of a compound containing a metal base of sulfonic acid and a polyethylene glycol. When used as a top coat or a coating agent, a polyester containing a large amount of polyethylene glycol hardly proves practical, due to the poor adhesion between the film coating and a substrate and the degraded appearance of the coating, such as reduced gloss, the occurrence of blisters and/or changes in hue, which are attributable to the absorption of water or moisture caused by a polyester having too high hydrophilicity. Even more impractical is the use of such a polyester as a primer, where corrosion resistance is required.
In order to improve the water resistance of a film coating of the aqueous dispersion described above, Japanese Patent Examined Publication No. 058092/1986 proposed an aqueous dispersion, wherein a polyester prepared from a polyalkylene glycol containing no aliphatic ether bond, such as polyethylene glycol, as a glycol component, and an aromatic dicarboxylic acid containing a metal base of sulfonic acid, are dispersed in an amphipathic solvent, as a means to achieve superior water dispersibility and water resistance. Although the resulting film coating is superior in water resistance, the film has insufficient resistance to boiling water, which requires higher resistance than does resistance to water below boiling temperature, and corrosion resistance, and is not entirely satisfactory in film appearance, e.g., smoothness and surface gloss. Japanese Patent Unexamined Publication No. 233127/1985 discloses a good aqueous dispersion obtained from a polyester comprising terephthalic acid and ethylene glycol by utilizing a metal base of sulfonic acid and terminal carboxylic group, but the film thereof lacks sufficient corrosion resistance as required of a primer for pre-coating. It should be noted that, when a glycol having high hydrophobicity is used as required of a resin for a primer, dispersibility or solubility in water is insufficient.
Accordingly, an object of the present invention is to provide an aqueous dispersion suitable for producing film coatings having superior processability and corrosion, boiling water, chemical and scratch resistance.